Field of Invention
This invention generally relates to a quantum rod backlight module used in liquid crystal display for enhancing the gamut and light utilization of the liquid crystal display.
Description of Related Art
Polarizers commonly used in the liquid crystal display are absorptive polarizers. In the liquid crystal display, when the non-polarized light emitted from the backlight is incident onto the absorptive polarizers, a component of the incident light parallel to the absorption axis direction of the polarizers is absorbed and cannot pass through the absorptive polarizers. Therefore, after the light emitted from backlight passing through the absorptive polarizer, the light will lose at least 50% in theoretically. In addition, after the light further passing through a polarizer, an electrode layer, a color filter, a liquid crystal module and a glass substrate, only less than 10% of the light, which originally emitted from backlight, can be transmitted to the observer. Accordingly, the light utilization of the backlight is quite low and causing waste of energy.
Several approaches to enhance the light utilization of the backlight have been proposed. For example, reflective brightness enhancement film (e.g., Dual Brightness Enhancement Film, DBEF), prism sheet and other optical film may be used in backlight module for continuously reflecting and reuse the light, which is unable to be transmitted by the polarizer, in order to redirected the light and let it pass the polarizers to enhance the brightness of the backlight module. In another example, concentrating the light of large viewing angle area of the liquid crystal display can increase the luminance at viewing direction. However, those solutions may increase the luminance of the backlight module; no significant effect is provided to the gamut and the color saturation of the liquid crystal display.
Therefore, a solution is proposed by using quantum dots in backlight module to increase the gamut thereof. The quantum dot is a semiconductor material of zero-dimentional structure, and it is able to absorb UV light or blue light having shorter wavelength and emit green light or red light having longer wavelength in order to mix to white light source. Because the spectrum of the excitation light of the quantum dot is with a narrower full-width-at-half-maximum (FWHM), the gamut of the liquid crystal display using the quantum dots will be more than 100% NTSC.
In addition, another approach is proposed that a quantum rod layer is integrated into the backlight module. The quantum rod is a nano-scale semiconductor material and the shape belongs to one-dimensional structure. The quantum rod is different from the absorptive polarizer, which absorbs non-polarizing light with evolution of heat. The quantum rod is able to absorb the non-polarized light to emit a polarized light with a wavelength longer than the original non-polarized light from the major axis direction thereof. Because of the high internal quantum efficiency, most of the incident light from the backlight source can be transformed to polarized light. The quantum rods are aligned in the direction of major axis, and the emitted polarized light is efficiently passed through the transmission axis of the polarizer disposed on the liquid crystal display. Accordingly, compared to the backlight module with quantum dots, the light utilization of a backlight module with the quantum rods will further be enhanced.
Usually, the dichroic ratio (DR) is used to evaluate the efficiency of transformed polarizing light emitted by quantum rod layer. The dichroic ratio is obtained by an equation DR=Y///Y⊥, wherein the Y// is the transmittance obtained as the major axis of the quantum rod layer is parallel to the transmission axis of the detection polarizer; Y⊥ is the transmittance obtained as the major axis of the quantum rod layer is perpendicular to the transmission axis of the detection polarizer. When a backlight source is not transmitted through a quantum rod layer, the Y// and Y⊥ are almost the same and thus the dichroic ratio is about 1, since there is no directionality of the general light. As the dichroic ratio is higher, the dichroism of the quantum rod layer is significant. When a light is transmitted through a quantum rod layer with a higher dichroism, the light will be transformed into a light with a better polarization and directionality. As using quantum rod layer in the stacked optical films of the current backlight module, the light reflection and the light refraction in the optical films, the retardations of the optical films or the light be scattered by the particles composed in films, decrease the dichroic ratio of the light excited by the quantum rod layer passing through these optical films. Thus, when the light generated from the backlight module with a quantum rod layer passes through the polarizers of the liquid crystal display, the brightness of the display is not as expected.
In addition, referring to FIG. 1, it shows the arrangement of a conventional backlight module. In a conventional backlight module, a set of a first micro-prism layer 11 and a second micro-prism layer 12 is included, wherein a plurality of first parallel strip-shape prisms 11a of the first micro-prism layer 11 and a plurality of second parallel strip-shape prisms 12a of the second micro-prism layer 12 are perpendicularly arranged for directing light at side-viewing angle to the forward viewing angle to increase the brightness thereof. However, when a quantum rod layer 13 is directly disposed in the arrangement of the conventional backlight module 1, i.e., the quantum rod layer 13 is disposed between the backlight source 14 and the first micro-prism layer 11, the dichroism of the quantum rod layer 13 declines because the difference of the retardation of the materials of the first micro-prism layer 11 and the second micro-prism layer 12 and the perpendicularly arrangement offset the polarization of the polarized light generated by the quantum rods 13a of the quantum rod layer 13. Thus, although the polarized light generated by the quantum rod layer 13 passes through the first micro-prism layer 11 and the second micro-prism layer 12 to be directed to increase the brightness at viewing angle, the polarized light is unable to maintain the direction of the polarizing axis to be consistent to the direction of the transmission axis of the polarizer disposed on the liquid crystal cell when subsequently passing through the polarizer. Thus, the polarization effect generated by the quantum rod layer 13 is unable to be utilized properly.